1. Field of the Invention
The present invention relates to a method of seismic prospecting with very high resolution in boreholes which are either horizontal or very highly inclined.
In the description which follows below, the expression "horizontal borehole" refers either to a borehole which is slightly inclined or not inclined at all with respect to the horizontal or else to a borehole which is highly inclined or deviated from a vertical wellbore.
2. Description of the Prior Art
It is an increasingly widespread practice to drill horizontal boreholes, especially in productive hydrocarbon deposits in order to drain the deposit to a distance which is considerably greater than its thickness whereas a vertical borehole drains the deposit only to a distance corresponding to its thickness. Within the deposit zone, a horizontal borehole cuts through geological strata at a small angle or even passes through the same stratum over an appreciable distance of the order of several hundred meters, the axis of said borehole being substantially parallel to the limits of said geological stratum.
The horizontal borehole drilled from a vertical well is usually of relatively substantial depth within the productive formation which is constituted by superposed strata separated by interfaces.
A study of the strata located above the horizontal borehole and therefore traversed by the vertical wellbore is very important in order to obtain information relating to lithologic properties (chemical compositions), sedimentologic properties and fracturing properties of rocks, fluid content and petrophysical parameters (porosity, permeability, compressibility, and so on).
A study of this type is carried out by means of the usual technique known as well-logging which may in particular be of the acoustic type. The acoustic values measured by means of an acoustic well-logging tool of the type described in French Pat. No. FR 2,431,710 (EVA.RTM. Process) are essentially the velocities and attenuations of the compressional waves and shear waves which are representative of the rock traversed by the acoustic paths and of the Stoneley waves and pseudo-Rayleigh waves which are representative both of the rock traversed and of the geometry of the wellbore.
The measurement is made by transmitting an acoustic wave through the fluid which fills the vertical wellbore from one or a number of transmitting sources and by receiving at one or a number of receivers waves of various types produced by the transmitted wave. The waves arriving at the receiver or receivers are converted to electrical signals by the receiver or receivers (of the piezoelectric type, for example). The electrical signals are transmitted to the surface by an electric cable, then recorded preferably in digital form in a suitable recording instrument. The recorded signals are then processed by a computer in order to identify the arrivals corresponding to each type of wave, especially compressional waves and shear waves and in order to determine the properties of interest such as propagation velocities (or arrival times), amplitude, period. Various processing operations may be performed in order to improve the accuracy and quality of the measurements to be made. Among those which have been proposed, one suitable processing operation described in French Pat. No. 81 18 672 is worthy of mention.
A distinguishing property of all well-logging measurements lies in the fact that they are performed within the vertical wellbore and that they relate to a small portion of space surrounding said vertical wellbore. This applies to all well-logging systems, whether they are of the acoustic, electrical, nuclear or other types. In particular, in the case of acoustic logs, this is due to the fact that the paths of the acoustic waves are paths of the refracted type. In fact, the waves transmitted through the vertical wellbore fluid are refracted at the wall of the wellbore, follow the wall over a certain distance, are then again refracted and finally arrive at the receiver or receivers of the logging tool.
The waves which arrive at the receiver or receivers are therefore practically those which have followed a path along the wall of the vertical wellbore. In consequence, the measurements concern only a small lateral portion of material of the geological stratum located between the transmitter or transmitters and the receiver or receivers of the well-logging tool. By "lateral portion of material" is meant the thickness of material traversed by the transmitted acoustic wave and considered in a direction substantially perpendicular to the vertical axis of the wellbore. It is readily apparent that the measurements made relate primarily to the thickness of the geological stratum as considered vertically and therefore the vertical portion of stratum located between the transmitter or transmitters and the receiver or receivers.
Although the transmitted acoustic waves propagate over considerable distances throughout the medium which surrounds the vertical wellbore, it has in fact proved impossible to receive and record reflected waves except in rare cases due to heterogeneities such as faults, fractures or salt domes, for example.
Up to the present time, formation analysts were unable to gain direct access to data relating to geological strata located beneath the horizontal borehole. In order to permit access to these data, it was necessary to carry out a very approximate study based on geological survey-map assumptions, dynamic measurements (pressure-flow relations) within the horizontal borehole or between boreholes (interferences), seismic surface measurements. Unfortunately, surface measurements have low selectivity and are uncertain, in the first place by reason of the presence of a modified zone at the ground surface and in the second place by reason of the considerable distance which has to be traversed by the waves between the surface and very deep geological strata. Apart from the large amount of energy to be produced in order to permit propagation of waves to the deepest geological strata, it is to be noted in addition that the accuracy of measurements is relatively low for the reason that the frequency employed has to be fairly low. In practice, measurements are accurate to within an approximation of a few tens of meters.
Moreover, data relating to strata located beneath the horizontal borehole are very difficult if not actually impossible to obtain by reason of the fact that these strata are too remote from the borehole for a traditional well-log.